The present invention relates to a driving device and more particularly relates to a driving device and a driving method in which electromechanical transducers such as piezoelectric elements (e.g., so called “piezo elements”) are utilized. The driving device of the invention is suitable, for example, as a driving system for optical components in a camera, DVD equipment, MD equipment, an endoscope and the like and a driving system for a precision stage.
As a driving device utilizing electromechanical transducers such as piezoelectric elements of which lengths are changed (extended and contracted) with application of voltage, for example, there has been known a driving device shown in an exploded perspective view of FIG. 1A and an assembly perspective view of FIG. 1B (see JP 11-98865 A).
The driving device, in which a movable body 10 can be moved relative to a base 1, can be used as a lens driving device for a camera, for example. That is, connection of the movable body 10 to a lens frame makes it possible to move a lens with the movable body 10.
A piezoelectric element 4 is composed of a large number of piezoelectric plates that are stacked. One end 4a of the piezoelectric element 4 with respect to a direction of the extension is fixed to the base 1, and the other end 4b is fixed to a first end 5a of a rod (a support member) 5. The rod 5 is slidably supported by support parts 2 and 3 that are formed integrally with the base 1.
The rod 5 is clipped by a main body 11 and a cap 12, a biasing force in the clipping direction is applied to the main body 11 and the cap 12 by a pressure spring (biasing member) 13, and thus the movable body 10 is engaged with periphery of the rod 5 by a frictional force. That is, the movable body 10 is supported by the rod 5 so as to be capable of sliding to the rod 5 by action of a force which overcomes the frictional force.
To the piezoelectric element 4 is connected a voltage control circuit not shown. When specified drive voltage having a sawtooth waveform is applied to the piezoelectric element 4, the piezoelectric element 4 vibrates with sawtooth displacement having generally the same form (see FIG. 2). Concomitantly, the rod 5 vibrates with the sawtooth displacement in a direction of a length thereof. A graph of FIG. 2 shows the vibration displacement of the piezoelectric element 4 and shows the vibration displacement of the rod 5.
In a mildly rising slant section 101 in a period A of a first waveform 100, specifically, the piezoelectric element 4 elongates comparatively slowly and the rod 5 moves slowly in a direction of an arrow I in FIG. 1B. In a period B, whereas, in a waveform part shown in a falling slant section 102, the piezoelectric element 4 rapidly contracts to an initial length and the rod 5 moves rapidly in a direction of an arrow II.
The same movement is thereafter repeated and, consequently, the rod 5 vibrates with repetition of the slow movement in the direction I and the rapid movement in the direction II. Thus the rod 5 vibrates while drawing such sawtoothed vibration waveform having slow parts and fast parts as shown in FIG. 2.
In the driving device, a spring force (i.e., a friction force of the movable body 10 with respect to the rod 5) of the pressure spring 13 of the movable body 10 is adjusted so that the movable body 10 moves with the rod 5 by the frictional force against the rod 5 when the rod 5 slowly moves and so that the movable body 10 stays through inertial force which overcomes the frictional force against the rod 5 when the rod 5 rapidly moves, as shown in FIG. 3. As a result, the movable body 10 moves in the direction 1 relative to the base 1 while the rod 5 vibrates.
In order to move the movable body 10 in the direction of the arrow II in FIG. 1B, the vibration waveform of the piezoelectric element 4 and the rod 5 shown in FIG. 2 has only to be reversed. That is, the wave form has only to have steep rising parts and gently sloped falling parts. A principle of the movement of the movable body 10 is the same as the above.
As described above, the drive voltage having the sawtoothed waveform is required to be applied to the piezoelectric element. As a method of producing such drive voltage, there has been known a method that will be described below.
<<Method Using Waveform Generator and Amplifier (see FIG. 4)>>
Sawtoothed waveform of 8 bits and 0-5 V is produced by D/A conversion performed by the waveform generator and is amplified to 0-10 V with use of a power amplifier (see FIG. 4A). Thus the sawtoothed waveform of 0-10 V for driving is obtained.
FIG. 4B shows waveform of drive voltage on occasion of a thrust of the movable body 10 in the direction I in FIG. 3, and FIG. 4C shows waveform of drive voltage on occasion of a drive in the opposite direction.
<<Method Using Constant-Current Circuit and Switching Circuit (See FIG. 5)>>
In a digital circuit shown in FIG. 5A, sections A and D configure constant-current circuits and sections B and C configure switching circuits. For the digital circuit, a signal shown in FIG. 5B is applied to terminals (a) through (d). Thus the sawtoothed waveform of 0-10 V for driving is obtained by alternate operation of the constant-current circuits A, D and the switching circuits B, C.
Both the methods described with reference to FIG. 4 and FIG. 5 are disclosed in JP 09-191676 A.
As described above, use of the waveform generator and the amplifier, use of the constant-current circuits and the switching means, and the like have conventionally been required for the obtainment of the sawtoothed drive voltage waveform. Such requirement leads to complicated configurations and cost increase.